G-Force Plot and Motorcycle Chains - Ask the Geek

Your Abnormal Guide To Things Abnormal

G-Forces graph

**I ♥ Acceleration
** Re. "The Traction Circle" (RSS, Sept. '09). Andrew, this is a great article which struck at the heart of the matter (pun intended), and possibly uncovered a deeper philosophical reason why we love riding so much-all those 3-dimensional G-forces plotted on a graph form a heart-a sign of love.

As to the technical information, I've been staring at the heart, errr.. graph for some time-slicing, dicing, and comprehending all the love, err.. information coming from it. Some of the conclusions are more important than others, but here is what I derived in no particular order.

1. It seems the riding was done in a clockwise direction at the track resulting in more right turns, hence the density of dots on the graph is noticeably higher on the right side.

2. The rider seems to be a bit more confident in right turns: a) the utmost right edge of the graph is slightly more away from the center compared to the utmost left edge of the graph suggesting more extreme lean angle in right hand turns; b) at partial lean angle (roughly half of maximum G-force), the rider applied both acceleration and braking a bit more aggressively on right hand turns than on left hand turns.

3. Now this is amazing. At decent lean angles (corresponding to 2/3 of maximum G-forces) the rider could use almost 100 percent of acceleration forces and about 1/2 of maximum braking forces! Draw two vertical lines on the graph-one 2/3 the distance to the left of the center and another 2/3 the distance to the right of the center, and see where they cross the higher and lower edges of the graph.

4. In the article, you pointed out that the dip in upright acceleration on top of the graph is caused by the limited motorcycle power not exceeding the traction limits. I think the ability to generate higher G-forces at slight lean angles can be attributed to the reduced effective tire radius at slight lean angles, which in turn shortens the gearing and increases the torque. As the lean angles increases, this effect is being gradually cancelled by less available traction for acceleration-hence the top of the graph in each half slightly peaks at first as we go away from the center, and then drops down.

5. Brakes are the most powerful control. Comparing the distances from the center to the top of the graph with the distance to the bottom of the graph reveals that maximum braking G-forces are about 50 percent stronger than maximum acceleration G-forces.

6. But what is more amazing is that maximum cornering G-forces are about 30 percent bigger than maximum braking G-forces. The former ones correspond to the distance from the center of the graph to the left and right edges. The latter one corresponds to the distance from the center of the graph to the bottom edge.

7. Now, this is questionable, but I swear if I look at the graph upside down, I see a sportbike going left to right. The bike is slightly leaned to the left, with the rear tire closest to the viewer. I think a drink or two would help to see it better.

On a serious note, the following things would be helpful.

a) put the units of measurements on the graph, and provide scale lines every 0.2g b) what bike and what tires were used, and who was riding it for that matter? c) provide us with the graph in the wet as well so we can compare dry vs. wet traction d) When you do tire comparisons, how about comparing different tires and providing two graphs for each tire-dry and wet?
Igor Gershteyn
Long Island, NY

This is an excellent analysis of the G-G plot and shows just how much information is available from a GPS-based data acquisition system. Here is a similar plot to what we ran in the article in question, with the axis labeled. This represents our man Kento riding clockwise at Buttonwillow on a literbike equipped with DOT race tires. G-forces depend not only on the tire characteristics, but also the track-bumps, camber, radius and altitude all affect how much lateral acceleration can be applied and can make the graph lopsided from left to right. Maximum braking is limited by the rear wheel coming off the ground as well as available traction (just as acceleration is limited by the engine's power as much as traction), explaining some of the differences top to bottom. I always have a couple of martinis before looking at data, it definitely helps.

Oops
After reading your article "Chains and Bearings" in your Ask the Geek section (Sept. '09), I have to say that adjusting a chain on most motorcycles is a sensitive thing to get done right. You never answered as to how much the chain should move under tension. Also as far as I know there should be the rider's weight on the bike while checking this. Most Suzuki bikes for example are very sensitive to adjust (like my TL). Also when tightening the rear axle (on most Suzukis), after adjusting the proper slack, the wheel seems to move in some way back and the chain is usually way too tight. Maybe another article to revisit this issue would be helpful for most readers and riders.
Matt Ander
Nelson, BC Canada

I didn't say in the answer how much the chain should move because it's different for each bike. What I failed to mention was that this number is almost always listed in your bike's owner's manual, sometimes along with more detailed information about how and where to measure-such as with or without the rider's weight on the bike. The amount of slack required is determined by the relationship between the front and rear sprockets and the swingarm pivot. Not knowing the measurement for slack, you can align the pivot and sprockets by having someone (it may take a heavy person or even two) sit on the bike; at this point of maximum tension, you still want a small amount of slack in the chain. The Suzuki GSX-Rs going back to the first SRAD models, along with your TL, are difficult because the chain tightens as you tighten the axle. With the axle loose, its threads are riding in the spacer; as you tighten the axle, its larger-diameter shoulder pulls into the spacer, pulling the wheel back and changing the chain tension. On these bikes it's a matter of accounting for this change when you measure the slack, or tightening the axle each time you check.

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